1. Field of the Invention
This invention relates to propellant storage and, more specifically, relates to densifying the stored propellant.
2. Description of the Related Art
Propulsion systems utilizing cryogenic propellants, such as liquid oxygen and liquid hydrogen, such as the Space Shuttle, Atlas/Centaur, Delta, etc., are currently filled from the facility storage tanks and subsequently allowed to cool in the flight tanks in order to reject the heat absorbed by the liquid as a result of environmental heat leak, transfer line, and tank wall chill-down. The cooling of the liquid bulk is desirable in order to increase the liquid density so that more impulse mass can be stored in the tank, and also to reduce the liquid vapor pressure so that the tank operating pressure and tank weight is minimized.
Heat rejection from the liquid bulk is a relatively slow process since it depends on natural convection mechanism and liquid surface evaporation. The degree of liquid cooling through surface evaporation is also limited by the vent system flow resistance (vent valve and line) and the ambient pressure (14.7 psia). Reducing the vent system flow resistance to improve the cool down time and minimize the final bulk temperature results in a relatively large vent valve and line design which represents a vehicle payload weight penalty. Although the current means of densifying the cryogenic liquids through evaporation are simple the process is limited to the saturation density and liquid vapor pressure at one atmosphere.
The prior art discloses systems that result in reduced weight of a vehicle with a propellant tank and reduced energy required to transport the vehicle. One such system is disclosed in U.S. Pat. No. 5,644,920 to Lak et al. entitled "Liquid Propellant Densification," which is incorporated by reference herein in its entirety.
The prior art also discloses the importance of maintaining propellant tank pressure before and during engine operation. Referring now to Prior Art FIG. 1, a prior art propellant tank 10 is partially filled with propellant 12 and has above the propellant an ullage 14. The tank 10 is filled with the propellant 12 through a transfer line 16 that directs the propellant from a heat exchanger/liquid filling system 18 to the bottom of the tank. The filling system 18 initially receives a propellant stream 20 to fill the tank 10.
The tank 10 has a manifold 22 for venting ullage gas from the tank 10 during the initial propellant charging process and drawing off warmer propellant 12 from the tank during the propellant densification process. The manifold 22 is shown in the upper half of the tank 10 and below the surface 24 of the propellant for densification. The manifold 22 is connected to a manifold line 26 that is also connected to the filling system 18. The manifold line 26 has a vent line 28 coming off of it. The manifold line 26 and the vent line 28 have valves 30 and 32, respectively, to control the flow of material through the lines. The valve 30 is located downstream of the tee for the vent line 28.
During the initial propellant charging process, the tank 10 is vented through the manifold 22. As the amount of the propellant 12 in the tank 10 increases, the ullage 14 decreases and needs to be vented. The ullage 14 is vented through the manifold 22 and out of the vent line 28. The manifold line valve 30 is closed and the vent line valve 32 is open to direct venting gas 34 out through the vent line 28. The venting through the manifold 22 continues until orifices 36 in the manifold are submerged, at which time the vent line valve 32 is closed. The initial propellant charging process continues until a predetermined full charge of propellant 12 is delivered to the tank 10.
After the initial propellant charging process, the propellant 12 is densified. The densification process involves removing propellant 12 from the tank 10, cooling the propellant, and directing it back into the tank. The manifold 22 is used to draw off propellant 12 and the propellant is directed through the manifold line 26 and the now open manifold line valve 30 and into the heat exchanger/filling system 18. The propellant 12 is cooled in the filling system 18 and directed back to the tanks 10 through the transfer line 16. The positioning of the manifold 22 in the upper portion of the tank 10 draws off propellant 12 that is warmer than propellant nearer the bottom of the tank.
After the densification process, the propellant tank 10 is pressurized. The tank 10 is isolated from the filling system 18 and the vent line 28. The propellant tank 10 is pressurized through pressurization gas 40 being introduced into the ullage 14 through a diffuser 42. The diffuser 42 is mounted in the tank 10 such that it is in the ullage.
The diffuser 42 is designed to direct the incoming pressurization gas 40 away from the propellant surface 24 and reduce heat transfer between the warmer gas and the cooler propellant 12. There are certain advantages to inhibiting heat transfer between the gas 40 and the propellant 12. The warmer pressurization gas 40 heats up the propellant 12, thereby detrimentally decreasing the propellant's density. The gas 40 cooled by the propellant 12 is denser, thereby requiring a detrimental increase in amount and weight of the pressurization gas tank to achieve the target tank pressure.
The pressurization gas 40 is directed to the diffuser 42 through a gas line 44. The gas line 44 has a gas line valve 46 that is closed during the initial propellant charging process.
The prior art design for a propellant tank requires two sets of piping, the manifold line 26 and the pressurization gas line 44. The prior art design also requires a manifold 22 for venting and recirculating and a separate diffuser for introducing pressurized gas 40 into the tank 10. A need exists for a system that combines the functions of the above items.